The present invention relates to a portal-type precision measuring apparatus, wherein the portal is adapted to translate on a platform and comprises a first cross-piece along which runs a carriage bearing a measuring head, and wherein the weight of the carriage is unloaded through resilient means on to a second cross-piece parallel to the first cross-piece and movable with it.
In a known measuring apparatus (which does not employ a second cross-piece) the deformations produced by the weight of the carriage on the first cross-piece are compensated by acting on the pressure of the air in the supporting bearings of the portal by means of valves controlled, in dependence upon the position of the carriage along the cross-piece, by cams. This solution, however, does not allow exact and complete compensation of the deformations because of the difficulties of reproducing the law of these deformations on the cams. Moreover, this is a relatively costly solution because of the regulating valves.
In another known apparatus, in which the cross-piece is carried by a pair of suitable guides, it has been proposed to unload the weight of the carriage on to a second cross-piece which requires two supplementary guides for the movement thereof. This measuring apparatus, however, is very heavy and costly because of the double guide.